Due to the recent tornadoes in Oklahoma, the word aftermath keeps cropping up as news reporters broadcast from the scene of all that destruction. So I began to be curious about the origin of the word aftermath, since it is 1.) kind of an odd word similar to understand (what does under + stand have to do with comprehension?), 2.) unique (we don't have words like beforemath, overmath, etc.), and 3.) has limited applicability.

I was certain that the -math had nothing to do with mathematics for the same reason cat has nothing to do with catalogue. But what then was its origin?

Well, this is from etymonline:

aftermath (n.)1520s, originally a second crop of grass grown after the first had been harvested, from after + -math, a dialectal word, from Old English mæð "a mowing, cutting of grass" (see math (n.2)). Figurative sense by 1650s.

And:

math (n.2)"a mowing," Old English mæð "mowing, cutting of grass," from Proto-Germanic *mediz (cf. Old Frisian meth, Old High German mad, German Mahd "mowing, hay crop"), from PIE *me- "to cut down grass or grain with a sickle or scythe" (see mow).

First recorded use is from 1520! That's a good long period of usage. And the math had to do with the cutting of grass. Who'd a thunk it!

This is clearly one of those words whose usage has drifted from the original meaning, like decimate has.

landlubber (n.) also land-lubber, sailor's term of contempt for a landsman, c.1700, from land (n.) + lubber (q.v.).

lubber (n.) mid-14c., "big, clumsy, stupid fellow who lives in idleness," from lobre, earlier lobi "lazy lout," probably of Scandinavian origin (cf. Swedish dialectal lubber "a plump, lazy fellow"). But OED suggests a possible connection with Old French lobeor "swindler, parasite," with sense altered by association with lob (n.) in the "bumpkin" sense. A sailors' word since 16c. (cf. landlubber), but earliest attested use is of lazy monks (cf. abbey-lubber). Cf. also lubberwort, the name of the mythical herb that produces laziness (1540s); and Lubberland "imaginary land of plenty without work" (1590s). Sometimes also Lubbard (1580s).